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TCA9517A: I2C signal undershoot

Yusuke Mukuno
Expert 7395 points
Part Number: TCA9517A

Hi Expert,

Customer have a question about I2C signal undershoot.

They found the undershoot on I2C SDA line at ACK timing, like as below,

It was over than -0.5 (abs max).

They check several board and several times.

It seems to be from -600mV to -750mV / 25ns to 35ns.

Customer tried to add dumping resistor, however it was not improve well.

Question1, Does the waveform undershoot have any issue on the TCA9517A?

Question2, The worst undershoot is -750mV/ 35ns on their system. is it any problem?

Question3, Do you have any idea for the acceptable range for this undershoot?

Thanks

Muk

  • 0
    TI__Guru 72521 points

    Hi Muk,

    Question1, Does the waveform undershoot have any issue on the TCA9517A?

    Since the undershoots are going past the absolute minimum spec, it may be a good idea to place schottky diodes on the pins that are seeing the undershoots. (GND to SDA and GND to SCL)

    Question2, The worst undershoot is -750mV/ 35ns on their system. is it any problem?

    I'll have to double check with our design team to see if this is a concern or not. 

    Question3, Do you have any idea for the acceptable range for this undershoot?

    I'll have to double check with our design team to see if this is a concern or not. 

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 7395 points

    Hi Bobby-san,

    Thank you for your supporting.

    Do you have any update? I want to get your answer by 9th Aug.

    Thanks

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Guru 72521 points

    Hi Muk,

    Here is what our designer has said:

    "Since this duration is so short and doesn’t appear to be occurring that often this should be fine as long as they’re not seeing excessive current coming out of the pin during the undershoots. In general, I would say as long as the voltage stays around the diode voltage ~0.7V and the duration is under 50nS or so this shouldn’t cause any damage.  Some exceptions may be if this was occurring on every single clock pulse or if they were seeing more than a few mA coming out of the pin during the undershoots, but outside of that it should be generally okay."

    In short, this should be okay. Placing some small series resistors (think 10 ohms or so) infront of our device may help to limit the current, I wouldn't make this resistor too large though since it will shift the VoL of the device higher. 

    -Bobby